1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly an apparatus for forming an image which is used in an electrophotographic process.
2. Description of the Related Art
An image forming apparatus, e.g. an electrophotographic copying apparatus, includes an original document table on which an original document to be copied is placed, an illumination device for illuminating the original document placed on the table, a rotatable photosensitive drum on which an image corresponding to the image information of the original document is formed, a charging device for charging the photosensitive drum with a desired charge, and a lens for stabilizing the image information supplied onto the photosensitive drum via the illumination device.
In the copying apparatus, the image information on an original document is illuminated by an illumination device and converted to a bright/dark pattern of reflection light corresponding to the image information. The converted information is transmitted onto a photosensitive drum via a lens. The photosensitive drum is rotated in a desired direction. On the other hand, the illumination device is elongated in a direction (scanning direction) perpendicular to the direction of rotation of the photosensitive drum. Accordingly, the image information on the original document is successively transmitted in accordance with the rotation of the photosensitive drum, in the state in which the information is divided substantially linearly in the scanning direction.
In addition, a copying apparatus of the above type, which can copy an image with a copying magnification varied, is known. In this copying apparatus, the distance between the exposure position and the photosensitive body varies as the optical system for varying the magnification moves and accordingly the length of an optical light path varies. As a result, the amount of light reaching the photosensitive drum differs between the enlargement copying mode and the reduction copying mode, and images with different densities may be formed.
In order to solve the above problem, Jap. Pat. Appln. KOKAI No. 4-19636 discloses a method in which data, which makes the amount of light of an exposure lamp in the enlargement mode greater that in the reduction mode, is pre-stored for each of magnifications, and the pre-stored data is referred to in accordance with the set magnification in order to control the amount of exposure, thereby preventing occurrence of a difference in density associated with the magnification.
The illumination device has a lamp extending in the scanning direction. The image information supplied from the original document onto the photosensitive drum is passed through a lens. It is known that a light intensity distribution based on "rule of cos.sup.4 " appears in the scanning direction between the intensity of light (light amount) traveling through a center portion of the lens and the intensity of light (light amount) traveling through a peripheral portion of the lens, i.e. light emitted from both end portions of the lamp.
In consideration of this, a light amount adjusting apparatus is built in many copying apparatuses midway along the light path from the illumination device to the photosensitive drum. The light amount adjusting apparatus functions to correct an intensity distribution provided by the "rule of cos.sup.4."
However, in a general light amount correcting apparatus, the correction amount is determined on the assumption that the magnification ("copying magnification") of a copied image in relation to the image on the original document (i.e. the ratio in size of the copied image to the original image) is 1 (1:1) ("100%-magnification copying mode"). Thus, if the copying magnification is less than 1 (reduction copy mode), the amount of light passing through a peripheral portion of the lens is greater than the amount of light passing through a center portion of the lens.
In the case where, like the prior art, the exposure amount is varied and controlled in accordance with the magnification in order to prevent occurrence of a difference in density due to the magnification, the aforementioned phenomenon (in which the amount of light passing through the peripheral portion of the lens becomes greater than that of light passing through the center of the lens) will occur.
This means that in the reduction-copy mode the luminous intensity on the photosensitive drum at an end portion in the scanning direction increases. Accordingly, as regards the copied image on the photosensitive drum, the latent image potential at an end portion in the scanning direction is lower than that at a center portion.
In this case, there is a problem concerning the image density of the reduction-copied image, that the density at both end portions is lower than that at the center portion. Further, in general, in the reductioncopied image, e.g. an image of a line drawing or a thin-line pattern, the image density tends to be attenuated in relation to development characteristics. Thus, when the image information on the original image contains many line drawings or thin-line patterns, an image defect may occur at both end portions of the copied image.